* Smarter generic entity macros
The list of "core" entities is now defined in one macro, and hardcoded
_Generic dispatch tables are eliminated
* Get rid of "custom" entities
All entities are now "first-class". The list of known entity types has
been moved to known_entities.h. The system no longer needs to know the
definition of all entity structs.
* Refactor guts of ENT_BOX/ENT_UNBOX
Made the functions inline, Box::ent is now a proper pointer type (but
please don't use it directly), ENT_UNBOX returns NULL if the box is
"empty" (references NULL entity)
* Merge TASK_BIND_UNBOXED with TASK_BIND
* s/YoumuMyon/YoumuAMyon for consistency
